Electric power transmission lines require regular inspections to ensure safety and reliability. Hazardous, expensive, and time consuming tower climbing inspections are typically used to verify the structural integrity of pole-tops, cross arms, and other elevated components. “Structural integrity” refers generally to its soundness, or, more specifically, to the absence of macro- and microstructural irregularities that are known or suspected to affect the strength of the material. In addition to the aforementioned deficiencies, tower climbing inspections are inconsistent and will vary from inspector to inspector.
Structural integrity can be tested by using destructive or non-destructive techniques. Material testing for quality control continues to be mostly destructive in nature despite efforts to develop non-destructive alternatives that are more feasible in terms of price, convenience and reliability. Although destructive testing is quite often more accurate because the condition of the material is made manifest rather than inferred. The obvious disadvantage is that the material or product tested is destroyed or rendered useless by the testing process. Furthermore, testing integrity by removal of already in-place structures, like cross arms on power lines, is not practical.
Alternatively, structural integrity can be tested using non-destructive techniques. Most non-destructive testing evaluates the material's composition and structure by relying on the interaction of the tested material with sound waves or electromagnetic radiation. Such methods involve monitoring the effect of pressure or electromagnetic waves passing through the material as they are influenced by flaws or inhomogeneities in the test structure. Monitoring the effects is typically done by making contact between the measuring device and the material.
Laser beams are known for use in non-destructive testing to detect structural defects. For example, a laser beam is projected onto a test object, the object is vibrated and the pattern of light reflected from the object is analyzed. As the frequency and intensity of vibrations are varied, changes appear in the pattern of light. Particular changes indicate that defects are present in the object. Non-destructive materials testing systems make use of the relationship between resonant frequency and the structural soundness of materials.
The analysis in most non-destructive testing of this type relies on the relationship between the material's resonant frequency and the strength and quality of the material's structure. The resonant frequency of a material depends upon, among other things, the material's shape, density, stiffness and the like.
Typically, the tested material structure is vibrated using a known force that is in contact with the structure (such as a hammer blow or vibrator exciting a power pole) and the vibrational characteristics of the tested area is measured. The collected data is used to compute the resonant frequency of the tested area. Generally, digital computers are used to perform evaluations based on the resonant frequency using known relationships. However, this method of creating vibration is time consuming and costly.
Acoustic resonance techniques have been used to measure the integrity of wood. Degradation can be determined by examining the acoustical resonance characteristics of wood. If there is an increase in the damping of the longitudinal acoustic waves, then the integrity of the wood has been degraded. However, a vibration generator must be attached to one point on the pole while a sensor is attached at another point on the pole. Performing this for the hundreds of thousands of transmission structures would be an arduous and expensive undertaking.
Another solution was to use the damping loss factor of a material to determine qualitatively the structural integrity of a material. The data analysis was performed using a standard digital analysis technique. As above, an electrodynamic shaker is attached to the pole to cause a vibration, while the vibration is measured with a laser vibrometer. Using this technique to determine structural integrity for the numerous transmission structures located in the United States would also be arduous and expensive.
Thus, there is a need to find an apparatus and method to measure structural integrity safely, remotely, accurately, and in an inexpensive manner.